When clear skies at night lead to a hard frost, growing media in pots can be frozen solid. Next day, plants in bright sunshine may need water, but can’t use it if it’s frozen. Those plants can become dry enough to deteriorate or even die. This post is about keeping them safe.

LOOK FOR DANGER

Check the bottoms of a few pots, to see whether they contain solid ice. Whenever you can see this, the growing medium above the ice is usually sufficiently water-logged for it also to contain ice, which will be very slow to thaw, even on a bright sunny morning. This can prevent plants from translocating enough free water to their upper parts, which become desiccated. Even a light breeze will accelerate this, leading to permanent wilting and “freeze drying,” just like instant coffee. I have known this do over £8,000 of damage in a small nursery in one morning.

PROTECTION

You need to prevent excessive transpiration until the root ball has thawed sufficiently. Fit your watering lance with a rose that delivers as fine a spray as possible. Open the supply valves fully, then turn the rose upwards, and use it to damp the foliage and stems lightly. In UK conditions you should try not to water the growing medium; the sun will thaw it faster if it is not water-logged. I suggest that in principle you should damp your plants over as soon as the problem is identified, and again last thing in the day, and again first thing in the morning. (Fruit growers use similar techniques to coat fruit buds with ice, because that prevents desiccation)